Conventional techniques use a wide variety of devices for determining ionic activities in solutions such as dry type ion-selective electrodes. The methods using these electrodes are determined from the veiwpoints of easiness in maintenance, storage and handling at measurement as well as of low cost. For example, rod-form or probe-form ion-selective solid electrodes are disclosed in U.S. Pat. No. 4,115,209, Japanese Utility Model Publication No. 14472/65, U.S. Pat. No. 3,718,569, etc. Also, ion-selective solid electrodes each composed of a support having formed thereon a laminate structure of four-functional layers, a laminate structure of three-functional layers, or a laminate structure of two-functional layers as film-shaped structure are disclosed in, for example, Research Disclosure, #16113 Sept., 1977), U.S. Pat. Nos. 4,053,381, 4,115,209, 4,214,968, Japanese Patent Application (OPI) Nos. 17851/82 and 17852/82, etc. (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). Such an ion-selective electrode is generally composed of a structure comprising an electrically insulating support having laminated thereon, in succession, a metal layer, a layer of a water-insoluble salt of the metal same as the metal of the aforesaid metal layer, a dried reference electrolyte layer composed of a hydrophilic binder matrix having dissolved therein a water-soluble salt having a common anion to the water-insoluble salt layer (dried reference electrolyte layer is omitted in the foregoing three-functional laminate layer type ion-selective electrode), and an ion selective layer. Furthermore, U.S. Pat. No. 4,115,209 discloses ion-selective electrodes of a two-functional layer laminate structure wherein a conductive material is coated with an ion-selective layer containing an ion-exchange material.
Devices for determining ionic activity using such ion-selective electrodes include a device composed of a pair of juxtaposed ion-selective electrodes and a capillary bridge (hereinafter, is simply referred to as bridge) for attaining electric conduction between the electrodes by being formed on these electrodes to electrically connect them. A determination method using the device is performed by spotting a test solution and a reference solution onto definite positions, respectively, of the electrodes to cause an electric conduction between both solutions through the bridge and measuring the potential difference occurring between the electrodes by means of a potential measuring device. That is, the solutions spotted on the surfaces of the electrodes wet the surfaces thereof and at the same time diffuse into the bridge disposed on the electrodes by a capillary phenomenon. The solutions are brought into contact with each other at a thin contact interface, whereby an electric conduction is attained between the electrodes to make it possible to measure the potential between the electrodes. In this case, each ion-selective electrode has an ion-selective layer selectively responsive to a predetermined specific ion as the outermost layer. The ion-selective layer is a layer which is generally composed of an ion carrier, a solvent for the ion carrier, and a polymer binder. The ion-selective layer can be selectively responsive to a specific ion by selecting the ion carrier and the solvent for the ion carrier. This means that in the case of determining the ionic activity of each of Na.sup.+, K.sup.+, Ca.sup.2+, Cl.sup.-, etc., in, for example, serum solid electrodes, each having a different specific ion-selective layer (the layer may be an ion-permeable protective layer described below in case of determining the ionic activity of Cl.sup.-) must be separately prepared.
The inventors previously proposed a device for determining ionic activity comprising plural pairs of juxtaposed solid electrodes each pair having a different ion-selective layer and a single porous bridge disposed on all the electrodes (Japanese Patent Application No. 82986/82 (corresponding to U.S. application Ser. No. 495,329, filed May 17, 1983)). One embodiment of the invention of the foregoing patent application is explained by referring to FIG. 1 and FIG. 2 of the accompanying drawings. That is, FIG. 1 is a plan view of the embodiment of the device for determining ionic activity of the foregoing patent application and FIG. 2 is a cross sectional view taken along the line I--I in FIG. 1. The device illustrated in FIG. 1 is composed of three pairs of solid electrodes 23 (23a and 23b), 24 (24a and 24b), and 25 (25a and 25b) each pair having a different ion-selective layer showing a selectivity for each specific ion and the solid electrodes having, for example, the laminated layer structures as shown in FIG. 2. The ion-selective electrodes are each composed of an electrically insulating support 1 having laminated thereon a conductive metal layer 2, a reference electrolyte layer 4 (the reference electrolyte layer may be omitted), and an ion-selective layer 5 (for the electrode pairs 23 and 24) or an ion-permeable protective layer (for the electrode pair 25). As is clear from FIG. 2, each pair of electrodes is electrically isolated from other electrodes. Furthermore, the electrodes in each pair of electrodes are also insulated from each other. Each pair of electrodes has terminals 13 for electric connection at both ends thereof, said terminals 13 being formed by exposing the metal layer (not shown) of the solid electrode at the end portions, and a potential measuring device (not shown) is brought into contact with the electrodes at the exposed portions through probes to measure a potential difference between the electrodes. The whole surface of the metal layers of the electrodes except the terminal portions 13 are covered by functional layers constituting the electrodes laminated on the metal layers. In another embodiment, in place of forming the terminal portions 13 by exposing the metal layers at the portions as described above, the whole surface of the metal layer is covered by the functional layers without exposing terminal portions and stylus probes of a potential measuring device may be brought into contact with the metal layers of the electrodes by piercing the stylus probes through the layers on the metal layers from the uppermost layer to measure the potential difference between the electrodes.
A single porous capillary bridge 19 is placed over all the solid electrodes for electrically connecting or conducting two individual electrodes in each pair of the solid electrodes. The bridge 19 has apertures 20 and 22 capable of applying each test solution and reference solution onto a definite position of the surface of each electrode in each pair. When the solutions are spotted onto these apertures, each of the solutions wets the surface of the electrode at the spotted portion and also permeates and diffuses into the bride 19 from the inside wall of the aperture. The porous layer of the bridge 19 is sealed at the edge portions 11 to prevent the solutions from being oozed from the edges of the bridge 19. The foregoing apertures have functions of enabling spotting of solutions to definite positions of the solid electrode surfaces and also as cavities for storing the solutions therein as will be understood from FIG. 2.
By spotting each of the test solutions and each of the reference solutions onto the surfaces of each pair of electrodes of the devices for determining ionic activity having the foregoing construction, the ionic activities of plural kinds of different ions each contained in each test solution can be simultaneously determined on plural items. According to the foregoing invention of our previous patent application, the determination of ionic activities can be greatly improved with respect to maintenance, storage, operation, etc.
Now, in the device for determining ionic activity provided by our previous invention described above, the function of attaining the electric conduction and the distribution of liquids, that is, the uniform distribution of test solutions and reference solutions to the surfaces of the electrodes are performed by using a sheet-form capillary bridge and hence both solutions must be spotted onto the surfaces of each pair of the electrodes in a short period of time. That is, it is necessary that the solutions spotted to the plural apertures diffuse into the bridge under the same condition to form a uniform contact interface between each pair of electrodes.